While exploring London in June of 2026, I decided to walk across Tower Bridge. Upon closer inspection, and further research, I discovered the bridge is made out of two main building stones: Cornish Granite (specifically from Cheesewring Quarry) and Portland Limestone (specifically from Perryfield Quarry). This earthcache will focus on Cornish Granite. For more information about the Portland Limestone, see this other nearby earthcache.

Most visitors to Tower Bridge admire its Victorian engineering, iconic towers, and panoramic views over the River Thames. Although Tower Bridge is often thought of as being "made of stone," its structural framework is actually steel. 

The granite forms part of the bridge's architectural cladding and surfaces, protecting and enhancing the engineering beneath.Yet beneath the decorative architecture lies a remarkable geological story that began over 280 million years ago, deep beneath what is now Cornwall.

The grey granite used in parts of Tower Bridge was quarried from southwest England and transported to London during construction between 1886 and 1894. This EarthCache explores the origins of Cornish granite, the geological processes that created it, why it was selected as a building stone, and how it has endured for well over a century in one of the world's busiest cities.

Geological Background

How Cornish Granite Formed

Cornish granite formed during the late stages of the Variscan (Hercynian) mountain-building event approximately 280–295 million years ago, during the Early Permian.

As two ancient continents collided, enormous pressures and temperatures developed deep within the Earth's crust. Existing rocks partially melted, producing silica-rich magma that slowly rose into the crust. Rather than erupting as volcanoes, these magma bodies cooled slowly several kilometres underground.

Because cooling occurred over millions of years, large mineral crystals had time to grow, producing the coarse-grained texture that characterizes granite.

Subsequent uplift and millions of years of erosion removed the overlying rocks, exposing these granite bodies at the surface across Cornwall, where they can be seen today as the Cornubian Batholith.

Minerals

Cornish granite is an intrusive igneous rock composed primarily of:

• Quartz – hard, glassy, translucent crystals that provide strength and weather resistance, easy to carve
• Orthoclase and microcline feldspar – usually white or pink, forming much of the rock and fills gaps of smaller grains
• Plagioclase feldspar – another important feldspar mineral where much of the binding strength comes from
• Biotite mica – dark brown to black flakes
• Muscovite mica – shiny silver flakes, common in many Cornish granites.

Accessory minerals may include:

• Tourmaline - varying colors with columnar shape that have parallel linear striations 
• Cassiterite - pyriamdial to tetrahedral shape with a silvery to deep grey color
• Wolframite - metallic grey color wiith blocky shape that have parallel linear striations
• Apatite - orange color with hexagonal shape
• Zircon - ruby red color with a prismatic shape
• Fluorite - bright, vibrant colors with a cubic shape

The mineral-rich fluids released during the final stages of granite crystallization created the famous tin and tungsten ore deposits that made Cornwall one of Europe's most important historic mining regions.

Why Was Cornish Granite Chosen for Tower Bridge?

Tower Bridge required a stone capable of enduring London's climate, pollution, vibration and constant pedestrian and vehicle traffic.

Cornish granite offered several advantages:

• extremely high compressive strength
• excellent resistance to weathering
• low water absorption
• durability against frost damage
• attractive light-grey appearance
• ability to be cut and dressed precisely

The granite was used in areas requiring exceptional durability, particularly where stone experiences continual wear.

What Makes Cornish Granite Geologically Special?

Cornish granite is unique because it represents one of the largest exposed granite batholiths in Britain.

The granite is notable for:

• exceptionally slow underground crystallization
• large, easily visible mineral crystals
• widespread mineralization by tin, tungsten, copper, arsenic and other metals
• extensive hydrothermal alteration
• its association with one of the world's classic granite-related mining districts

Few granites have contributed so much to both geological research and human history.

The Quarry

The granite used for the cladding of Tower Bridge was quarried from Cheesewring Quarry, situated on Stowe's Hill near the village of Minions on the eastern edge of Bodmin Moor in Cornwall.

Commercial quarrying began in the mid-1800s after a lease was granted in 1845. The arrival of the Liskeard and Caradon Railway allowed large blocks of the distinctive silver-grey granite to be transported from the moor to the port of Looe, where they were shipped to London and other destinations around Britain.

Cheesewring granite became one of Britain's premier construction stones during the Victorian era. In addition to Tower Bridge, it was used in the Thames Embankment, Westminster Bridge, Lambeth Bridge, dock works, breakwaters, and many other major engineering projects because of its exceptional strength, durability, and attractive appearance.

The quarry lies immediately below the famous natural granite tor known as the Cheesewring. Quarrying expanded so close to the tor during the nineteenth century that there were concerns the landmark might be destroyed by blasting. Public pressure eventually led to the protection of the Cheesewring, and quarrying later declined before finally ceasing in the twentieth century.

What makes this Cornish Grantie unique from other granites?

One of the characteristics that helped make Cheesewring Quarry granite desirable as a building stone is its porphyritic (megacrystic) texture. Rather than having crystals that are all about the same size, many blocks contain noticeably larger white feldspar (plagioclase) crystals (called megacrysts) surrounded by a finer-grained mixture of quartz, feldspar and mica. 

These large feldspar crystals began growing while the magma was still deep underground before the remainder of the magma completely crystallized. As cooling continued, the surrounding minerals filled in around them, producing the distinctive texture still visible today.

Another distinctive feature of many Cornish granites is the presence of black tourmaline, a boron-rich mineral that formed during the final stages of the granite's crystallization.

As the granite magma slowly cooled deep underground, most of the common rock-forming minerals—quartz, feldspar, and mica—crystallized first. Elements that did not readily fit into these minerals, including boron, became increasingly concentrated in the remaining molten rock and the hot, water-rich fluids dissolved within it.

When these fluids became sufficiently enriched in boron, tourmaline began to crystallize, often forming long, dark prismatic crystals within the granite or along small mineral-filled fractures. This process occurred during the final stages of cooling, after much of the granite had already solidified.

The abundance of boron-rich fluids is one reason the Cornubian granites are famous not only for their tourmaline, but also for the hydrothermal systems that produced Cornwall's historic deposits of tin, tungsten, copper, and other metals. Although not every block of Cheesewring granite contains visible tourmaline, where present it provides evidence of the chemically evolving, mineral-rich fluids that accompanied the final stages of granite formation. This can lead to a range of different appearances to tourmaline, resulting in beautifully colored crystals.

Notes about logging/finding this earthcache

To log this cache you need to be below the bridge, on the south side, where the granite really shows it's true natural form. If you are walking above the rbidge, there is a staircase on the south side that leads right to ground zero. You will need to walk these steps for your required photo and for your observations. The example photo below shows where you will need to be:

TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.

NOTE: It is discouraged to use AI in your response, and AI often generates wrong or misleading answers. So it is best if you avoid using it all together. All you need is your observations, along with the description, to answer all the questions.

1. "Cornish Granite at Tower Bridge" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.

2. Take a photo of you at the bottom of the stairs (or your signature item if you don't want to show your face) with the granite wall and staircase behind you (at the posted coordinates). Note you must have both the granite wall and stairs in your photo. See example photo above for the correct location and to confirm what I am looking for. This MUST be uploaded to your log at the time you submit your log. No two logs may contain the same photo!

3. Look at the rough cut (un-polished) Cornish Granite along the wall of the stairs. Tell me the range(s) of (a) textures, (b) colors, (c) grain sizes, and (d) if there is any weathering/erosion occurring.

4. Look at the smooth cut (polished) Cornish Granite along the wall of the stairs. Tell me the range(s) of (a) textures, (b) colors, (c) grain sizes, and (d) if there is any weathering/erosion occurring. (e) How does this compare with your answers in question 3? Why might your answers be different? Hint: think about geologic processes that lead to Cornish Granite forming.

5. Looking at either the rough cut or polished granite blocks, look for one of the primary minerals: plagioclase feldspar. Then describe its physical qualities such as its (a) shape(s), (b) range of color(s), and (c) average grain size(s).

6. Looking at either the rough cut or polished granite blocks, look for one of the accessary minerals: tourmaline. Then describe its physical qualities such as its (a) shape(s), (b) range of color(s), (c) general orientation (i.e. are the grains alligned or randomly distributed) and (d) grain size compared with plagioclase crystals (i.e. smaller, larger, or about the same).

7. (a) Is it easier to find both primary and accessory minerals in the rough cut or polished granite blocks? (b) Explain why you might expect a difference for certain minerals, but not others. Hint: think about the 3d shape of these minerals relative to the texture of the blocks. Does it matter?

8. (a) Do the crystals of all minerals appear to be evenly sized throughout the rock, or do you observe two noticeably different crystal sizes? Explain what this tells you about how the granite cooled underground. In other words, (b) did the cooling rate change or was it constant? If it changed, was it fast-to-slow or slow-to-fast? How can you tell?

9. Which mineral present in the granite would classify as megacryst? Explain your answer. Hint: which is the largest mineral present?

10. Noting all your previous answers, why do you think Cornish Granite was chosen for the outer shell of Tower Bridge? What are some geologic properties that you observed that make it desirable in construction?

Works Cited

https://www.caradonstone.co.uk/product/cornish-granite-sawn-walling-stone/

https://www.tandfonline.com/doi/full/10.1080/03090728.2020.1731247

https://scienceviews.com/photo/library/SIA3669.html

https://www.open.edu/openlearn/science-maths-technology/geology/geological-processes-the-british-isles/content-section-4.6

https://depositsmag.com/2026/03/21/a-history-of-the-plate-tectonics-of-britain-part-1-britain-assembled-oceans-collisions-and-the-making-of-a-geological-patchwork/

https://www.buildingconservation.com/articles/rockofages/rockofages.htm

https://exploringtheearth.com/2013/07/29/gradschooltasmania/olympus-digital-camera-31/